It is a well-known fact that low-frequency sound waves can travel longer distances through water than can high-frequency sound waves. For a long time there has also been a considerable need of powerful low-frequency sound transmitters which are capable of working under water, both from a military point of view and from the point of view of the offshore oil and gas industry. Transmitters of various designs and embodiments for these purposes and fields of use have been available on the market for quite a long time. A summary of such acoustic transmitters is given in an article in DEFENSE SYSTEM REVIEW, November 1984, pages 50-55, entitled "Sonar transducer design incorporates rare earth alloy".
Most acoustic transmitters which are used at present are based on either the piezoelectric effect or on magnetostriction. As is well-known, the piezoelectric effect means that a crystalline substance is subjected to a change in shape when an electric voltage is applied to its end surfaces and that a voltage is obtained when the substance is subjected to a physical deformation, respectively. Magnetostriction means that a magnetic material which is subjected to a change of the magnetic flux suffers a change in shape and that an externally caused change in length gives rise to a change in the magnetic flux, respectively. This means that a transmitter which utilizes these effects can also, in principle, be used as a receiver.
A variety of different embodiments of acoustic transmitters exist. In low-frequency applications it is common that they have a cylindrical shape with either a circular or elliptical cross section area.
The greatest problem with this type of transmitters is to achieve a sufficiently great amplitude of the oscillations. To this end, either a large transmitter area or a small transmitter area with great amplitude of oscillation would be required.
The introduction of the so-called giant magnetostrictive materials has improved the conditions for obtaining good acoustic transmitters. With such materials as driving elements, amplitude changes may be obtained which largely amount to 30 times the corresponding changes using piezoelectric materials. Transmitters which utilize these giant magnetostrictive materials have existed for several years. One property of transmitters which utilize giant magnetostrictive materials is that they must be mechanically prestressed. This can be done in different ways, for example as shown in U.S. Pat. No. 4,438,509 with the aid of prestressed wires.
A frequently occurring embodiment for the actual driving will be described in greater detail starting from a cylindrical transmitter with a near elliptical cross section. The cylindrical envelope surface consists of an elastic diaphragm or shell. Inside and parallel to the axis of the cylinder and making contact with the shell are two beams applying pressure to the shell. The cross sectional area of the beams is symmetrically mirror-inverted in relation to the minor axis of the elliptical shell and each beam is delimited by that part of the shell which faces the end of the major axis and a chord parallel to the minor axis. Between the beams and making contact with their plane-parallel sides there is arranged an electrically-controlled driving element in the form of a driving rod. The longitudinal axis of the driving rod coincides with the major axis of the elliptically-formed cross section and lies midway between the end surfaces of the transmitter. In those case where the magnetostrictive effect is utilized, the driving rod consists of a magnetostrictive material which with a surrounding winding is magnetized to keep pace with the desired frequency of the transmitter. If the piezoelectric effect is to be utilized, the driving rod consists of a piezoelectric material. The driving rod may, of course, consist in its entirety, or in certain parts, of a material with the desired possibilities of changing the length.
The fundamental embodiment of an acoustic transmitter described above may be different as regards the actual details. An acoustic transmitter with a cylindrical shape and with an elliptical cross section area and with driving rods of a giant magnetostrictive material is disclosed, inter alia, in U.S. Pat. No. 4,901,293 entitled "A rare earth flextensional transducer".
Swedish patent 8901905-3, "Device in acoustic transmitters", also describes a cylindrical transmitter with elliptical cross section. The driving element here consists of a body with oppositely located recesses into which driving rods are inserted. The driving rods, in turn, are fixed into pressure rods which in the same way as above influence the diaphragm.
Swedish patent application 9003086-7 describes a drive package for acoustic transmitters comprising a frame of magnetic material with windows for mounting driving members and prestress devices. Two windows with driving members and an intermediate window with a mechanical prestress device form a column which, by means of pressure studs in the driving members and holes in the frame, prestress pressure beams, located inside the transmitter, in the shell of the transmitter. The drive package may comprise several columns.
The building system embraced by the invention comprises magnetic circuits for magnetization of the active material in accordance with U.S. Pat. No. 4,914,412, "Magnetic circuit". The magnetic circuits included are intended to magnetize cylindrically shaped pellets of magnetostrictive material, in the axial direction in accordance with the U.S. patent. This magnetic circuit comprises a magnetizing coil, disc-shaped permanent magnets for bias magnetization and discs of soft-magnetic material which have a diameter corresponding to the outside diameter of the coil as well as a soft-magnetic cylindrical tube which surrounds the magnetizing coil. The soft-magnetic parts are included in the magnetic circuits which comprise the magnetostrictive pellets.